The present invention relates to a technique for improving the startup of a power control loop at start of transmission in an electronic part for high frequency power amplification (RF power module) including a high frequency power amplifier circuit. In particular, the invention relates to the above technique that is effectively applied to the RF power module for use in, for example, mobile phones for GSM networks.
A transmitter output section of a wireless communication apparatus (mobile communication apparatus) such as a mobile phone has an electronic part for high frequency power amplification (hereinafter referred to as RF power module) having built therein a high frequency power amplifier circuit (PA) with a transistor such as a MOSFET (Metal Oxide Semiconductor Field-Effect-Transistor) or a GaAs-MESFET as an amplifying element.
A mobile communication system is generally configured such that a mobile phone during a call with another one changes its output power (transmitting power) adaptively to its ambient environment, according to information indicating a proper power level sent from a base station, thus preventing interference with other mobile phones. For example, a mobile phone of GSM (Global System for Mobile Communications) is configured as follows: it employs an Automatic Power Control (APC) circuit that compares an output signal detected with an output level directive signal Vramp from a baseband circuit and generates a control voltage Vapc to control the transmitting power and the gains of amplification stages of a high frequency power amplifier circuit in its transmitter output section are controlled by a bias control circuit to produce an output power required to continue a call in accordance with the control voltage Vapc (Patent document 1).
For a mobile phone for GSM employing a conventional RF power module, when the output level directive signal Vramp supplied from the baseband circuit rapidly rises to a transmitting power level at start of transmission, the output power of the high frequency power amplifier circuit rises at an excessively high rate, which causes a part of the spectrum characteristic of its output signal to fall outside the range prescribed by GSM standards. Specifically, the GSM standards prescribe that, in the spectrum characteristic of an output signal, the signal strength must be lower than its profile plotted by a dashed line A shown in FIG. 11(A). However, a problem was found in the actual output signal profile plotted by a solid line B; that is, the signal strength exceeds the prescribed profile in the foot of the rising waveform that represents the characteristic.
To address this problem, as is illustrated in FIG. 2 (A), a technique is disclosed (non-patent document 1), whereby, for a period (t1 to t5) after the power-on of the RF power module until the output level directive signal Vramp starts to rise, as preparation for transmission, a “precharge” operation (t4 to t5) that raises the voltage Vramp (Vapc) to a level equivalent to −25 to −30 dBm of output power and holds it for a short period of time such as 15 to 17 μsec is performed by a software process run on the baseband IC.
[Patent document 1] Japanese Unexamined Patent Publication No. 2000-151310
[Non-patent document 1] Data Sheet “SKY77324: iPACTM PAM for Quad-Band GSSM/GPRS” published by SKYWORKS